1. Field of the Invention
The present invention relates to a color filter usable for a display device such as a liquid crystal display, a display device using the color filter, and the method for manufacturing the color filter and the display device.
2. Description of the Prior Art
In recent years, because of outstanding features in terms of light weight, thin configuration, low power consumption, drivability with a low voltage, and the least physical influence on human body, practically applicable range of the liquid crystal display (this will merely be referred to as an LCD hereinafter) device has quickly been expanded. In particular, applicable field of the color LCD devices has sharply been expanded as the one most compatible with the updated color display for personal computers and also for a wide variety of those sophisticated apparatuses compatible with multi-media in trend.
In the field of a variety of color LCD display devices practically being available for various uses in the industrial field, according to the classification in terms of the display mode and the driving method, the one belonging to the “active matrix” type (this will merely be referred to as AM hereinafter) applying the twisted nematic (TN) mode and another one belonging to the “multiplex” type applying the “super twisted nematic (STN)” mode dominantly share the trend respectively. In addition, a wide variety of LCD driving methods have also been proposed, thereby promoting production of the color LCD display devices among the concerned industries based on the diversified display modes and driving methods.
The above-cited LCD modes “TN” and “STN” individually share the identical principles in the field of color display. Concretely, individual pixels are split into a number of dots corresponding to three primary colors. Next, by way of properly controlling voltage added to an LCD layer in each of the split dots, light permeability per dot is properly controlled. As a result, a certain color synthesized from individual three primary colors having controlled light permeability becomes a specific color to be displayed in the corresponding pixel. Normally, the three primary colors are red (R), green (G), and blue (B). Even in the case of adopting other driving methods, the principles of color display remains in common with each other, in other words, in common with the above-cited TN and STN modes.
In order to enable one of the three primary colors corresponding to an individual dot to selectively permeate through it, a color filter (CF) is applied. The color filter is disposed on a surface facing the liquid crystal layer of one of the two substrates made of glass and constituting the LCD. In the active matrix (AM) type LCD, generally, the color filter is disposed on the surface of an opposite substrate devoid of the provision of thin-film transistors (TFT) or diodes (MIM). In the case of the LCD based on the above-cited “super twisted nematic (STN)”, the color filter is provided on either of a pair of stripe-form substrates.
Constituents of the LCD are Described Below.
[1] Constitution of the Color Filter:
A certain number of colored layers each colored with one of the three primary colors (R, G, and B) are formed on the color filter CF. Further, in order to shield light, a black matrix BM is formed on the gaps between each of the colored layers, on those portions requiring prevention of light from leakage and also on the margin of the display area.
There is a general method for forming colored layers and a black matrix BM by initially forming the black matrix on a glass substrate followed by a step of forming colored layers on the black matrix. As another method, initially, colored layers are formed on a glass substrate, and then, a black matrix is formed so as to bury gaps between the colored layers.
Then, after completing formation of the colored layers and the black matrix, there is such a case in which an over-coating layer OC is formed on the colored layers and the black matrix so as to fully level off the surface of the color filter. Nevertheless, additional provision of the over-coating layer obliges the coating process to incur much load and poor yield, thereby causing the process for manufacturing the color filter to result in the substantial increase of production cost. From the standpoint of mass production of color filters, it is preferred that the process for forming the above-cited over-coating layer be deleted by all means.
In the following step, in order to drive the LCD display device, transparent electrodes are formed on those layers laminated on the glass substrate. The transparent electrodes are constituted with a compound of indium tin oxide (ITO). In the case of forming a TFT incorporated LCD display device, ITO patterns are formed all over the surface of the LCD display device. In the case of forming the above-referred diode incorporated LCD display device or the LCD display device based on the “super twisted nematic (STN)” mode, stripe patterns are formed in common with each other.
[2] Constitution of the Black Matrix:
To constitute a black matrix, metallic material such as chromium or black resinous material is used. When metallic material is used, because of toxicity of chromium, application of nickel and tungsten formed into a dual-layer constitution mainly prevails recently. In this constitution, a nickel layer is disposed on the display side, whereas a tungsten layer incorporating an extremely high reflection factor is disposed on the part of array. It is essential that the material of the black matrix be provided with a minimum of approximately 3 of optical density (OD) value in consideration of light shielding effect. In order to secure the required optical density, in the case of applying metallic chromium, a minimum of approximately 0.1 μm of film thickness is required. In the case of applying black resinous material, a minimum of 1 to 2 μm of film thickness is required.
In recent years, relative to a market tendency in which metallic tantalum element thus far mainly used for composing thin film transistors and diodes has become rarely procurable and quite expensive, practical use of aluminum increasingly prevails because of its low resistance value, inexpensive cost, and high reflection factor. However, due to multiple reflections caused by aluminum and the material of the black matrix which has an extremely high reflection factor, divergence is caused in characteristics. To cope with this problem, it is urged to lower the reflection factor of the black matrix on the part of the color filter, and in response, there is a progress in the arrangement for lowering the reflection factor of the black matrix since. Viewing from proper characteristics, it is desirable to use black resinous material for composing black matrix so as to meet the demand for lowering reflection factor of black matrix. In contrast with 60% of the reflection factor of metallic chromium, the reflection factor of black resinous material is merely 1 to 3%. Further, reflection spectrum of black resinous material is less dependent on the wave-length, and yet, exhibits neutral black shade. On the other hand, black resinous film is composed with 1 to 2 μm of substantial thickness, and thus, it is likely that this thickness could adversely affect the levelness of the surface of the color filter as a potential problem.
In order to lower the reflection factor, there is another method of applying black matrix composed of a laminate of a chromium-oxide layer and a metallic chromium layer or a laminate of a nickel layer and a tungsten layer. However, in this case, compared to the black matrices composed of black resinous material, either of the above laminate layers exhibits a reflection factor of 3 to 5%, which is higher than that of the black resinous material, and yet, instead of neutral black shade, the above laminate layers contain bluish purple shade as another problem. Further, normally, metallic double layers are processed via a sputtering method in the course of forming films, and thus; this process causes the productive efficiency to be lowered to result in the rise of production cost as another disadvantageous problem.
[3] Method of Forming Resinous Black Matrices:
There are a variety of practical methods for forming black matrices with black resinous material on a glass substrate. Typical examples are cited below.
[Method 1]
First, a glass substrate is superficially coated with negative light sensitive black resinous material so as to form a thin film thereon. This process can be implemented by coating the substrate surface with the black resinous material via a spin coater, or by adhering a piece of black resist material previously formed into a film onto the surface of the glass substrate, or by applying cascaded coating processes. Next, the surface of the glass substrate is irradiated with UV rays via photomasks having a predetermined black-matrix pattern to cause the exposed portion of the black resinous material to be hardened. Then, by removing unexposed portions of the black resinous material via a developing process, black matrices are thus formed eventually.
[Method 2]
First, as was performed for the method 1, negative light-sensitive uncolored resinous material is spread over the surface of a glass substrate so as to form a thin film. Next, as was performed for the method 1, the resinous film is exposed and developed so as to shape pattern of the original black matrix. Next, the pattern formed portion is colored with black shade by applying a non-electrolytic plating method or a dyeing method for example.
[Method 3]
First, as was performed for the method 1, black resinous material compatible with a developing process is spread over the surface of a glass substrate. Next, positive photo-resist pattern is formed on the surface of the material, and then, as was performed for the method 1, exposing and developing processes are serially executed. In the course of the developing process, photo-resist and black resinous materials are jointly removed. Next, by applying a thermal treatment, the black resinous material is cross-linked and hardened. Finally, unexposed resist component is removed.
[4] Formation of Colored Layers:
First, colored pigment is dispersed into resinous material in advance. Then, the resinous material containing dispersed pigment is spread over the surface of a glass substrate so as to form a thin film. In the next step, the thin film is patterned into a predetermined form by applying a photo-lithographic method (this will be referred to as the “pigment dispersion method” in the following description). Colored layers can also be formed by applying any of those methods including the following: a method which initially spreads light-sensitive resinous material over the surface of a glass substrate so as to form a thin film and pattern the film into a predetermined form and then colors the patterned film: a method which initially causes color pigment to be dispersed into resinous material and then prints the colored resinous material into a predetermined pattern on the surface of a glass substrate (this will be referred to as the “printing method” in the following description): a method which initially causes pigment and resinous material to be jointly dispersed in solution and then forms a predetermined pattern on a glass substrate via an electro-deposition process: a method which previously forms colored resist material into a thin film and then bond the colored thin film onto the surface of a glass substrate (this method is called the “dry film lamination (DFL)”: and a coloring method by applying an ink-jet coloring system. Since the object of the present invention is to overcome those problems existing in the DFL method, the following description will solely refer to the DFL method.
In the course of forming colored layers by applying the DFL method, it is so arranged that, initially, a thermal treatment is applied to a glass substrate patterned with the resinous black matrix material, and then, thermally adheres pasting material consisting of film-form colored layers stripped of a cover film on one side. This processing step is called a laminating process. FIG. 1 schematically exemplifies the laminating process, in which the reference numeral 1 represents a pasting roller, 2 a pasting material, and 3 represents a glass substrate. In the laminating process, as shown via an arrowed symbol A, the pasting roller 1 is shifted in one direction. Next, the other side cover film (not shown) on the colored layers is stripped off. Then, the colored layers are subject to an exposing process by applying a collective exposure mask or a stepper system, and then, organic components are removed via a post-exposing process. Finally, the colored layers are treated with a post-baking process.
Even when forming the black matrix resinous material via the pasting process performed in the method 1, the laminating process shown in FIG. 1 is introduced. In this case, resinous black matrix material prepared in the film form is used to function as the pasting material 2.
Normally, when manufacturing the color filter based on the above-referred DFL (dry film lamination) method, formation of the resinous black matrix material is preceded by the formation of the colored layers comprising a red layer, a blue layer, and a green layer. This sequence prevents foaming from being generated at the interface between the glass substrate, the resinous black matrix material, and the colored layers. The cause of the foaming will be described later on. When inserting the resinous black matrix material into the gap between individual colored layers, such a method is applied, which causes the back surface of resinous black matrix material to be exposed to harden. As described earlier, at least a minimum of the value 3 is required for the optical density. Further, inasmuch as users strongly demand in recent years that back-light luminance be enhanced furthermore, in order to enable the black matrix to properly shield light, it is desired to further secure reliable material capable of providing a minimum of the value 4 of optical density.
However, when causing the resinous black matrix material to be exposed from the back side to harden, unless the available material contains a minimum of the value 4 of optical density and reduces height difference from the colored layers, disorder in orientation will be observed at the junction between the resinous black matrix material and the colored layers. Principles of the observable orientation disorder are shown FIG. 2, in which the reference numeral 3 represents a glass substrate, 4 a source wiring installed in the TFTs, 5 an insulating film, 6 a pixel electrode, 7, a liquid crystal layer, 8a, 8b, 8c are respectively a colored layer each being colored with different colors, and 12 represents resinous black matrix material having a certain number of stripe-form openings, through which the above colored layers 8a, 8b, and 8c with a striped formation are respectively exposed. As shown in FIG. 2, when a user's eyes incline from the direction perpendicular to the LCD screen, user can observe plural portions beneath the resinous black matrix material 12 via the lateral surfaces of the colored layers 8a, 8b, and 8c. Disordered orientation appears in these portions 9.
To compensate for this faulty phenomenon, as in the case of applying the “pigment dispersion method” cited above, when forming the colored layers 8 after formation of the resinous black matrix material 12, the color layers 8 are superposed on part of the resinous black matrix material 12 to cause the black matrix portion to be concealed totally, thereby correcting the disordered orientation. FIG. 3 exemplifies the principles for preventing users from observing the disordered orientation by way of implementing the above method. Since the resinous black matrix material overlaps the edges of the colored layers 8a, 8b, and 8c, the user will no longer observe the portions 9 via lateral surfaces of the colored layers 8a, 8b, and 8c, thereby fully concealing the disordered orientation by means of the resinous black matrix material 12 and the source wiring 4 on the array side.
According to the previous invention duly disclosed in the Japanese Laid-Open Patent Application No. H09-105809, in order to prevent levelness from being lost by the swollen portion of the colored layers 8 on the resinous black matrix material 12 in the course of forming the colored layers 8 by applying the above-referred “pigment dispersion method”, it is so arranged that, by providing edge of the mask regulating external shape of the colored layers with fine waveforms, the rise of the edge of the colored layer 8 is gradual.
On the other hand, when applying the above DFL (dry film lamination) method, as shown in FIG. 1, the pasting material for constituting the colored layers is adhered in one direction by operating a pasting roller 1, when the pasting material 2 overleaps wall of the resinous back matrix material (not shown), foaming is apt to be generated at the interface between the substrate 3, the resinous black matrix material, and the pasting material 2. The aspect related to the generation of foaming is shown in FIG. 4, in which foaming 11 is generated at the interface between a side surface on the upstream side of the pasting direction A of the resinous black matrix material 12, the upper surface of the substrate 3, and at the bottom surface of the pasting material 2. Once the foaming 11 has ever been generated, the pasting material 2, i.e., the colored layers 8 complete with patterning via exposure and development, are discolored, thus resulting in the degraded quality of the produced image.